CN115478815A - Exploitation method, system and application of natural gas hydrate in warm high-pressure reservoir in sea area - Google Patents
Exploitation method, system and application of natural gas hydrate in warm high-pressure reservoir in sea area Download PDFInfo
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- CN115478815A CN115478815A CN202211165729.5A CN202211165729A CN115478815A CN 115478815 A CN115478815 A CN 115478815A CN 202211165729 A CN202211165729 A CN 202211165729A CN 115478815 A CN115478815 A CN 115478815A
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- NMJORVOYSJLJGU-UHFFFAOYSA-N methane clathrate Chemical compound C.C.C.C.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O NMJORVOYSJLJGU-UHFFFAOYSA-N 0.000 title claims abstract description 52
- 238000000034 method Methods 0.000 title claims abstract description 52
- 239000007789 gas Substances 0.000 claims abstract description 91
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Natural products C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims abstract description 31
- 239000003345 natural gas Substances 0.000 claims abstract description 16
- -1 natural gas hydrates Chemical class 0.000 claims abstract description 16
- 238000005553 drilling Methods 0.000 claims abstract description 13
- 238000006073 displacement reaction Methods 0.000 claims description 30
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- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 1
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/01—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells specially adapted for obtaining from underwater installations
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/16—Enhanced recovery methods for obtaining hydrocarbons
- E21B43/166—Injecting a gaseous medium; Injecting a gaseous medium and a liquid medium
- E21B43/168—Injecting a gaseous medium
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/25—Methods for stimulating production
- E21B43/26—Methods for stimulating production by forming crevices or fractures
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B7/00—Special methods or apparatus for drilling
- E21B7/04—Directional drilling
- E21B7/046—Directional drilling horizontal drilling
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Abstract
The invention discloses a method, a system and application for exploiting natural gas hydrates of a sea area warm and high-pressure reservoir, wherein the method comprises the following steps of drilling one or more groups of double horizontal wells in a hydrate reservoir; performing staged fracturing on each group of the double horizontal wells; and (4) alternately carrying out depressurization and gas replacement on the double horizontal wells after the staged fracturing. The invention is suitable for typical sea area warm and high pressure reservoir conditions: raw gas hydrate reservoir temperature>277K and pressure>8MPa; compared with the prior art, the development rate of the same reservoir condition is improved by more than 30 percent; final CH 4 Recovery and CO 2 The sealing rate exceeds 50 percent, and effectively realizes CH 4 Recovery of and CO 2 The double benefits of sealing;the cementing structure of the reservoir in the natural gas hydrate exploitation process is dynamically maintained, and potential environmental and reservoir safety risks caused by low reservoir strength and structural deformation are avoided.
Description
Technical Field
The invention relates to the technical field of natural gas hydrate exploitation, in particular to a method and a system for exploiting natural gas hydrates of a sea area warm high-pressure reservoir and application of the method and the system.
Background
The natural gas hydrate is a clean low-carbon fossil energy with wide distribution, high density and large resource quantity, and is a cage-shaped structure consisting of water molecules and natural gas molecules (99 percent is CH) under the conditions of low temperature and high pressure 4 ) Together forming an ice-like crystalline compound.
The depressurization decomposition method is a relatively efficient and low-consumption method, but it decomposes the hydrate into CH 4 Water, weakens the cementing structure of the argillaceous silt reservoir, is very easy to cause geological disasters such as seabed landslide, well wall collapse and the like, and induces large-scale greenhouse gas CH 4 Gas leakage; in contrast, CO 2 -N 2 The replacement process being by CO 2 And N 2 Molecular replacement of CH in hydrates 4 The guest molecules maintain the cementing structure of the reservoir, can relieve potential environmental and reservoir safety risks, and can promote CO 2 But slow rate of displacement and CH in the middle and late stages of development 4 The problem of low efficiency of recovery ratio is not broken through. Depressurization method and CO 2 -N 2 The combined mining method of displacement method is considered to solve the sea area CH 4 One of the contradictory technical schemes of 'efficient development' and 'environmental safety' in hydrate development is expected to 'take the strong points and avoid the short points' and improve CH 4 Recovery ratio, dynamic restoration of reservoir stability and realization of CO 2 And (4) sealing and storing the geology.
However, as shown in FIG. 1, the existing research and patent relate to hydrate production technology, and the oriented hydrate reservoir conditions are mostly concentrated at low temperature (C:)<277K) And low pressure (<6 MPa), while the typical sea reservoir is mostly in the middle-high temperature region (>277K) And high pressure (>8 MPa), for example, the temperature of a GMGS2-8 hydrate reservoir in the east sand sea area is 278.0-286K, and the pressure is 8.3-9.1 MPa; the temperature of the SH7 hydrate reservoir in the sea area of the Hovenia procumbens is 286.4-287.3K, and the pressure is 13.6-13.8 MPa. The natural gas hydrate exploitation method suitable for the sea area warm and high-pressure reservoir conditions is less in research. Further, as shown in FIG. 2, FIG. 2 is a heterogeneous region of the hydrate (CO in the illustration) 2 /N 2 /CH 4 In a proportion of hydrationMolar ratio of gas phase molecules in a phase equilibrium state), the thermodynamic phase equilibrium state of hydrates at different reservoir temperatures and pressures is different, and CO is easily introduced under low-temperature and low-pressure conditions 2 Hydrate steady state region and CH 4 Hydrate non-steady-state region (e.g. region a in fig. 2), favoring decomposition and displacement, but warm high-pressure reservoir conditions while CH is simultaneously 4 Hydrate, CO 2 Hydrate, and part of CO 2 -N 2 -CH 4 The stable phase region (e.g., region E in FIG. 2) of mixed hydrates, in the absence of thermodynamic regulation, may exhibit CH 4 Hydrate and CO 2 -N 2 -CH 4 Inhibiting CH formation due to large amount of mixed hydrate 4 Decomposition and replacement of the hydrate. Literature reports (Niu et al, chemical Engineering Journal,2021,420 4 The recovery ratio is less than 20 percent. Thus, under warm high pressure reservoir conditions in the sea, existing depressurization decomposition and CO 2 -N 2 The combined exploitation method of replacement does not really solve the bottleneck problems of efficient exploitation and environmental safety of the natural gas hydrate.
In particular, chinese patent CN103603638B discloses a natural gas hydrate CO combined with depressurization method 2 The displacement mining method comprises the steps of firstly adopting low-density mud drilling to reduce pressure of a natural gas hydrate reservoir and decompose part of natural gas hydrate; then the prepared CO is 2 Injecting the emulsion into the reservoir to enhance CO 2 Mass transfer and promotion of CO 2 Displacement reaction with natural gas hydrate. The invention is characterized by a pressure reduction method and CO 2 Single-round combined mining of the displacement method. Another characteristic is in CO 2 After the emulsion is injected into the reservoir, the reservoir pressure is controlled to be 3-5MPa, which is not typical pressure condition of the sea hydrate reservoir. Chinese patent CN113107433A (application publication number) discloses a method for reducing blood pressure and CO 2 The method for replacing and exploiting the natural gas hydrate comprises the steps of firstly adopting a depressurization exploitation mode to exploit free gas in a natural gas hydrate reservoir stratum; then the hydrate reservoir temperature is lowered and maintained below freezing point (<273.15K) to freeze free water in the reservoir; when reservoir temperature and pressureAfter the force is stable, exploiting the natural gas hydrate; injecting CO into the reservoir after natural gas hydrate production is finished 2 Carrying out CO 2 And (7) burying. The invention is characterized in that the temperature of the reservoir is 272.15K and the pressure is 1.0-3.0MPa in the process of extracting the natural gas hydrate, which is not typical of the temperature and pressure conditions of the sea hydrate reservoir.
In view of the above reasons, the invention fully considers the warm and high pressure conditions of the natural gas hydrate reservoir in the sea area, and adopts the existing depressurization method and CO 2 Metathesis, single-pass reduced pressure decomposition and CO 2 -N 2 On the basis of the advantages and the disadvantages of the replacement combined mining method, the development is inherited, and a multi-round decompression decomposition and CO decomposition are provided 2 -N 2 The displacement exploitation of natural gas hydrate.
Disclosure of Invention
The invention aims to provide a method, a system and application for exploiting natural gas hydrate of a sea area warm high-pressure reservoir, and aims to break through single-round depressurization and CO (carbon monoxide) under the condition of the sea area warm high-pressure reservoir 2 The technical bottleneck of the replacement combined mining method provides multiple rounds of decompression decomposition and CO 2 -N 2 The key technical characteristics of the technology for replacing and exploiting the natural gas hydrate are thermodynamic path dynamic matching and dynamic driving force bidirectional strengthening. In order to achieve the purpose, the invention provides the following technical scheme:
a method for exploiting natural gas hydrates from a warm and high-pressure reservoir in a sea area comprises the following steps of drilling one or more groups of double horizontal wells in a hydrate reservoir; performing staged fracturing on each group of the double horizontal wells; and (3) alternately carrying out depressurization and gas replacement on the double horizontal wells after the segmented fracturing.
Preferably, the hydrate reservoir comprises a sea area warm and high pressure hydrate reservoir, wherein the sea area warm and high pressure hydrate reservoir has a temperature of more than 277K and a pressure of more than 8MPa.
Preferably, the double horizontal well comprises an upper horizontal well and a lower horizontal well, wherein,
the upper horizontal well is arranged at the top of the hydrate reservoir, and the lower horizontal well is arranged at the bottom of the hydrate reservoir.
Preferably, the step of alternately performing depressurization and gas replacement on the bi-level well after the staged fracturing comprises,
depressurizing the double horizontal wells after the staged fracturing to a preset gas replacement condition;
injecting replacement gas into the lower horizontal well, exhausting the upper horizontal well, and replacing to obtain methane gas;
when the methane obtained is below a first predetermined concentration and the displaced gas is above a second predetermined concentration, or the partial pressure of the displaced gas in the reservoir is increased more than the displaced gas CH in the reservoir 4 When the phase equilibrium pressure of the mixed hydrate is 20 percent, primary pressure reduction and gas replacement are completed;
repeating the steps of reducing the pressure and replacing the gas for N times alternately, wherein N is an integer more than 1.
Preferably, the predetermined gas displacement conditions include a flow rate of the wellhead gas and CH 4 Concentration below a predetermined value, average CH 4 The gas partial pressure is reduced to below 10 percent of the natural gas hydrate phase equilibrium pressure corresponding to the average temperature or the average temperature is reduced by 3-5K compared with the initial temperature of the pressure reduction of the round.
Preferably, the fractured double horizontal well comprises a measuring device;
the measuring device comprises sensors for temperature, pressure, flow and the like, and is used for monitoring the thermodynamic state of the hydrates at the top and the bottom of the reservoir.
Preferably, the displacement gas comprises CO 2 And N 2 Mixed gas of CO 2 And N 2 In the range of 20: 80-50 parts of.
A marine warm high pressure reservoir natural gas hydrate production system, the system comprising,
the drilling module is used for drilling a double horizontal well in the hydrate reservoir;
the fracturing module is used for performing staged fracturing on the hydrate reservoir through the double horizontal wells;
and the displacement module is used for alternately carrying out pressure reduction and gas displacement on the double horizontal wells subjected to staged fracturing.
Preferably, the fracturing module comprises a depressurization unit and a monitoring unit, wherein,
the depressurization unit is used for carrying out step depressurization on the double horizontal wells to a preset gas replacement condition;
the monitoring unit comprises a temperature sensor, a pressure sensor and a flow sensor and is used for monitoring the flow of the wellhead gas, the methane concentration, the temperature and the pressure of the hydrate reservoir and evaluating the average CH in the hydrate reservoir 4 Gas partial pressure and reservoir average temperature.
The method for exploiting the natural gas hydrate in the warm high-pressure reservoir in the sea area is applied to exploitation of the natural gas hydrate reservoir in the warm high-pressure condition in the sea area.
The invention has the technical effects and advantages that:
1. the method is suitable for reservoir conditions of typical sea area warm and high pressure: the original natural gas hydrate reservoir temperature is greater than 277K and the pressure is greater than 8MPa;
2. compared with the prior art, the development rate of the same reservoir conditions is improved by more than 30 percent;
3. final CH 4 Recovery and CO 2 The sealing rate exceeds 50 percent, and effectively realizes CH 4 Recovery of and CO 2 The double benefits of sealing;
4. the cementing structure of the reservoir in the natural gas hydrate exploitation process is dynamically maintained, and potential environment and reservoir safety risks caused by low reservoir strength and structural deformation are avoided;
5. the method is suitable for reservoir conditions of typical sea area warm and high pressure: the original natural gas hydrate reservoir temperature is greater than 277K and the pressure is greater than 8MPa;
6. alternative development of reduced pressure mining and CO 2 -N 2 Displacement mining to form decompression-displacement \8230anddecompression-displacement mining sequence, i.e. multiple rounds of decompression and CO 2 -N 2 A displacement combined mining method;
7. based on the technical ideas of thermodynamic path dynamic matching and kinetic driving force bidirectional strengthening, adjacent pressure reduction operation and CO are regulated and controlled 2 -N 2 Gas injection displacement operation to synergize hydrate phasesThermomechanical and kinetic development. Wherein the dynamic matching of the thermodynamic path is characterized in that the development path of thermodynamic states such as reservoir temperature, pressure, gas phase partial pressure and the like is dynamically regulated and controlled to match with the initial CH 4 CO in hydrate to development process 2 -N 2 -CH 4 The mixed hydrate phase equilibrium line continuously and safely achieves the thermodynamic conditions of hydrate decomposition and replacement in each production run. The bidirectional enhancement of the kinetic driving force is characterized by timely introduction of CO in the middle and later low-efficiency stage of depressurization or replacement 2 -N 2 The replacement or pressure reduction process continuously utilizes the sequence of pressure reduction-replacement \8230andpressure reduction-replacement to strengthen the heat and mass transfer of the former (namely bidirectional strengthening).
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.
Drawings
FIG. 1 is a comparison of hydrate reservoir temperature and pressure conditions as related to prior studies and patents versus typical sea warm high pressure reservoir conditions;
FIG. 2 is a multiphase region of hydrates (CO in the legend) 2 /N 2 /CH 4 The proportion is the molar proportion of gas phase molecules in a hydrate phase equilibrium state);
FIG. 3 is a flow chart of a method for producing natural gas hydrates from a warm high-pressure reservoir in a sea area;
FIG. 4a is a schematic view of a dual horizontal well being drilled in accordance with an embodiment of the present invention;
FIG. 4b is a schematic illustration of hydraulic fracturing in an embodiment of the present invention;
FIG. 4c is a schematic illustration of reduced pressure mining in an embodiment of the present invention;
FIG. 4d is a representation of CO in an embodiment of the present invention 2 -N 2 A displacement mining schematic;
FIG. 5 is a schematic diagram of slow mass transfer across phases in the late stage of depressurization to restrict hydrate decomposition according to an embodiment of the present invention;
FIG. 6 is a schematic diagram illustrating the displacement of gas injection to enhance gas-water migration and accelerate hydrate decomposition according to the embodiment of the present invention;
FIG. 7 shows the decomposition under reduced pressure causing surface CO 2 -N 2 -CH 4 CO strengthening by mixed hydrate decomposition 2 -N 2 Deep replacement of (2);
FIG. 8 shows multiple rounds of depressurization and CO decomposition 2 -N 2 A schematic diagram of a thermodynamic path in displacement exploitation of natural gas hydrate;
fig. 9 is a schematic diagram of a sea area warm high pressure reservoir natural gas hydrate production system of the present invention.
In the figure: 1. an overburden; 2. a lower overburden formation; 3. a hydrate reservoir; 4. horizontal wells; 5. a fractured fracture; 6. a fracturing pump truck; 7. an air (liquid) pump; 8. CO 2 2 -N 2 The mixed gas is injected into the pump.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to solve the defects of the prior art, the invention discloses a natural gas hydrate exploitation method for a sea area warm and high pressure reservoir, which comprises the following steps of drilling one or more groups of double horizontal wells in a hydrate reservoir, as shown in figure 3; performing staged fracturing on each group of the double horizontal wells; and (4) alternately carrying out depressurization and gas replacement on the double horizontal wells after the staged fracturing.
In one embodiment of the invention, as shown in fig. 4a, a bi-level well is drilled in a hydrate reservoir by a directional drilling technique, wherein an upper horizontal well is close to the top of the hydrate reservoir and a lower horizontal well is close to the bottom of the hydrate reservoir;
in one embodiment of the present invention, as shown in fig. 4b, the hydrate reservoir is fractured in sections by a horizontal well fracturing technique, so as to improve the fluid seepage capability of the hydrate reservoir. In one embodiment of the invention, staged fracturing is to separate each group of horizontal wells into a plurality of well sections through a downhole separation tool, firstly, fracturing work is carried out on the first well section, and fracturing work on the rest well sections is carried out in sequence after the well section is finished, so that fine reservoir reconstruction with outstanding pertinence and good controllability can be realized through staged fracturing.
In one embodiment of the invention, measurement equipment including temperature, pressure, flow rate, etc. sensors are separately run in the bi-level well after completion to monitor the thermodynamic state of hydrates at the top and bottom of the reservoir.
In one embodiment of the present invention, as shown in fig. 4c, the dual horizontal wells are stepped depressurized by the pump to avoid the throttling effect of rapid depressurization resulting in abrupt reservoir temperature drop. And the pressure is gradually transmitted to a hydrate reservoir through a high flow guide channel formed by fracturing the fracture, so that the pressure reduction decomposition exploitation of the natural gas hydrate is realized.
In one embodiment of the invention, the step depressurization mode is depressurization-pause- \8230anddepressurization-stop, wherein the pause stage is used for promoting the uniform evolution of the reservoir pressure and avoiding the local too-fast depressurization.
In one embodiment of the invention, the flow rate and CH of the wellhead gas are monitored 4 Concentration, and hydrate reservoir temperature and pressure, estimating average CH in hydrate reservoir 4 Gas partial pressure and reservoir average temperature. When the flow rate and CH 4 Too low a concentration, or average CH 4 And (3) when the gas partial pressure is reduced to below 10% of the natural gas hydrate phase equilibrium pressure corresponding to the average temperature, or the average temperature is reduced by 3-5K compared with the initial pressure reduction temperature of the round, the air (liquid) pump is closed.
In one embodiment of the invention, the lower horizontal well head CO is opened as shown in fig. 4d 2 And N 2 Injecting mixed gas into lower horizontal well by pump 2 And N 2 Gas, CO 2 And N 2 Depending on reservoir conditions, may vary from 20: preferably between 80 and 50. CO 2 2 And N 2 Gas transmission through lower horizontal well fracturing fractureInto hydrate reservoirs. Meanwhile, an air suction pump of the upper horizontal well is opened to produce CH 4 A gas. In this step, as shown in FIGS. 5 and 6, CO, on the one hand 2 And N 2 Gas displaces free water in a hydrate reservoir to cause gas-water migration and accelerate CH 4 The cross-phase mass transfer speed of molecules in free water weakens the mass transfer limiting effect of natural gas hydrate decomposition; on the other hand, CO 2 And N 2 Gas further reduces CH 4 The partial pressure of the gas increases the decomposition driving force of the natural gas hydrate and promotes the natural gas hydrate to be continuously decomposed; finally, CO 2 And N 2 Gas-promoted metathesis, CO 2 And N 2 Entering a cage-shaped structure of the natural gas hydrate to exchange partial CH 4 Molecule, finally gradually replacing part of natural gas hydrate into CO 2 -N 2 -CH 4 A mixed hydrate. Therefore, in the injection of CO 2 And N 2 In the process, not only can the decomposition of the natural gas hydrate be maintained, but also CO is promoted 2 And N 2 Replacement, retention of the bond strength of hydrate reservoirs, and partial CO 2 Sealing and storing.
In one embodiment of the invention, the CO is injected 2 And N 2 In the process, the flow and CH of the wellhead gas are continuously monitored 4 /CO 2 /N 2 Concentration, temperature and pressure in hydrate reservoir, and estimation of CH in hydrate reservoir 4 /CO 2 /N 2 Average gas partial pressure, average reservoir temperature, CO formed 2 -N 2 -CH 4 Average proportion of guest molecules in the mixed hydrate. When producing CH 4 Too low concentration and CO 2 Too high a concentration, or CO in the reservoir 2 And N 2 Partial pressure rise over CO in the reservoir 2 -N 2 -CH 4 And (3) closing the air injection pump and the air (liquid) extraction pump when the phase equilibrium pressure of the mixed hydrate is more than 20%.
In one embodiment of the invention, the wellhead suction (fluid) pump is turned on, and a second round of depressurization is performed on the dual-level well in substantially the same manner and under substantially the same conditions as for the first round. Except that, as shown in FIG. 7, in the second placeBefore the first round, the hydrate reservoir only contains natural gas hydrate, and after the first round of replacement, the natural gas hydrate in the surface layer and the partial middle layer has CH with cage-shaped structure 4 Molecular quilt of guest gas CO 2 And N 2 By displacement to form CO 2 -N 2 -CH 4 The mixed hydrate is depressurized in the second and later rounds to promote CO wrapped by the surface layer of the undecomposed natural gas hydrate 2 -N 2 -CH 4 Partial ablation and loosening of the mixed hydrate are beneficial to breaking the shell layer of the compact mixed hydrate to CO 2 And N 2 Promote CO in subsequent replacement runs 2 -N 2 Deep replacement of (2).
In one embodiment of the invention, the steps after the fractured double-horizontal well is put into the measuring equipment are repeated, and the 2 nd to n th CO injection rounds are alternately carried out 2 -N 2 Gas exploitation and decompression exploitation constitute decompression-replacement 8230and decompression-replacement sequence. In the process of multiple times of mining, the key is to estimate CO in a hydrate reservoir by monitoring the flow rate and the concentration of each component of wellhead gas and the temperature and the pressure in the hydrate reservoir 2 /N 2 /CH 4 Average gas partial pressure, average reservoir temperature, CO formed 2 -N 2 -CH 4 The average proportion of guest molecules in the hydrate is mixed, and then the thermodynamic states such as reservoir temperature, pressure, gas partial pressure and the like are continuously regulated and controlled through depressurization and gas injection operations. FIG. 8 shows multiple rounds of depressurization and CO decomposition 2 -N 2 Schematic diagram of thermodynamic path in displacement production of natural gas hydrate (CO in illustration) 2 /N 2 /CH 4 The proportion is the molar proportion of gas phase molecules in a hydrate phase equilibrium state), the regulation and control aim is to realize 'thermodynamic path dynamic matching', and the concrete expression is to promote gas phase CH to be dynamically matched in a decompression regulation and control stage 4 The thermodynamic state of (A) is reduced to the time CO 2 -N 2 -CH 4 Under the equilibrium line of mixed hydrate phase, in the stage of displacement regulation, the gas-phase CO is promoted 2 And N 2 The thermodynamic state of the molecule is raised to the current CO 2 -N 2 -CH 4 Above the phase equilibrium line of the mixed hydrate, and then in each run of depressurization and gas injectionTo the thermodynamic conditions of decomposition and displacement of hydrates. Meanwhile, the multi-round combined mining also emphasizes the bidirectional enhancement of the dynamic driving force, which is particularly characterized in that CO is introduced in time at the middle and later stages of depressurization or replacement 2 -N 2 The replacement or pressure reduction process continuously utilizes the sequence of pressure reduction-replacement 8230and pressure reduction-replacement to strengthen the heat and mass transfer of the former and continuously overcome the attenuation of the kinetic driving force of the phase change of the hydrate. Finally, the small guest molecule N is superimposed 2 The natural effects of the stabilizer and the promoter in the depressurization stage of each turn continuously stimulate the phase change of the hydrate, and finally ensure that the natural gas hydrate is gradually changed by CO 2 -N 2 -CH 4 And (4) replacing by a mixed hydrate.
The invention also discloses a sea area warm high-pressure reservoir natural gas hydrate exploitation system, as shown in fig. 9, the system comprises a drilling module for drilling a double horizontal well in a hydrate reservoir; the fracturing module is used for performing staged fracturing on the hydrate reservoir through the double horizontal wells; and the displacement module is used for alternately carrying out pressure reduction and gas displacement on the double horizontal wells subjected to staged fracturing.
The fracturing module comprises a depressurization unit and a monitoring unit, wherein the depressurization unit is used for carrying out stepped depressurization on the double horizontal wells to a preset gas replacement condition; the monitoring unit comprises a temperature sensor, a pressure sensor and a flow sensor and is used for monitoring the flow of the wellhead gas, the methane concentration, the temperature and the pressure of the hydrate reservoir and evaluating the average CH in the hydrate reservoir 4 Gas partial pressure and reservoir average temperature.
The invention also discloses a method for exploiting the natural gas hydrate in the sea area warm high-pressure reservoir, and application of the method in exploitation of the natural gas hydrate reservoir under the sea area warm high-pressure condition.
The technical solution of the present invention will be further described with reference to specific examples.
And 4, continuing to perform depressurization and replacement development, wherein the depressurization and replacement operation of the later round is not simple repetition of the conditions of the first round of depressurization and replacement operation, but follows the academic ideas of 'thermodynamic path dynamic matching' and 'bidirectional enhancement of kinetic driving force' of a multi-round combined mining method, namely developing the regulation and control matching of the depressurization and replacement thermodynamic path and the thermodynamic phase equilibrium dynamic of hydrate in the reservoir, and performing the enhancement of the kinetic driving force on the latter through the former in a depressurization-replacement-8230-depressurization-replacement sequence. Specifically, the judgment basis of the final state condition of the subsequent turn of depressurization is as follows: wellhead gas flow and CH 4 The concentration is too low; the average temperature of the reservoir is reduced by 3-5K compared with the initial temperature of the pressure reduction in the current round; or CH in reservoir 4 The average gas partial pressure is reduced to below 10% of the natural gas hydrate phase equilibrium pressure corresponding to the average temperature. At the same time, the last round of replacementThe judgment basis of the state condition is as follows: output CH 4 Too low concentration and CO 2 Too high a concentration; or CO in the reservoir 2 And N 2 Partial pressure rise over CO in the reservoir 2 -N 2 -CH 4 The phase equilibrium pressure of the mixed hydrate is more than 20 percent.
And 5, opening an air (liquid) pump to perform second-round staged pressure reduction. In the second pressure reduction process, CO is generated due to the previous replacement stage 2 Hydrate and CO 2 -N 2 -CH 4 The reservoir strength is slightly enhanced by the cementing effect of the mixed hydrate, so that the reservoir temperature in the final depressurization state in the current round can be slightly lower than that in the first depressurization final state. Thus, in the second round of depressurization, when wellhead flow rate and CH are equal 4 Too low concentration, or average reservoir temperature falling below 276K, or average reservoir CH 4 When the partial pressure is reduced to below 3.1MPa, the pump is turned off and the production is stopped. It is noted that in the second drawdown development, in addition to monitoring total bottomhole pressure, CH should also be monitored 4 The gas partial pressure. In addition, the second round of depressurized decomposed hydrates includes natural gas hydrates (CH) 4 Hydrate), CO 2 Hydrate, CO 2 -N 2 -CH 4 Mixed hydrates, but due to N 2 The 'stabilizer' action of the molecule in the mixed hydrate, and the residual CO in the reservoir 2 -N 2 -CH 4 The mixed hydrate maintains the mechanical stability of the reservoir.
And 7, opening the air (liquid) pump to perform the third round of staged pressure reduction. When wellhead flow rate is equal to CH 4 Too low concentration, or average reservoir temperature falling below 275K, or average reservoir CH 4 When the partial pressure is reduced to below 3.0MPa, the pump is turned off and the production is stopped.
And (5) repeating the steps 4-7, and further carrying out depressurization and replacement mining of the 4 th round and the 5 th round.
Final CH 4 The recovery ratio is about 55 percent, the reservoir gradually recovers the initial temperature of 280K and the pressure of 8MPa, and the average guest molecule CO of the mixed hydrate in the reservoir 2 ：N 2 ：CH 4 The molar ratio is 50:5.5:44.5, realization of CH 4 Harvesting, CO 2 The triple benefits of sealing and repairing the reservoir stratum are achieved.
The invention alternately develops depressurization exploitation and CO 2 -N 2 Replacement mining to form depressurization-replacement 8230and depressurization-replacement mining sequence, i.e. multiple rounds of depressurization and CO 2 -N 2 A displacement combined mining method; based on the technical ideas of thermodynamic path dynamic matching and dynamic driving force bidirectional enhancement, adjacent pressure reduction operation and CO are regulated and controlled 2 -N 2 Gas injection displacement operation to synergize hydrate phase transition thermodynamics and kinetic development. Wherein, the dynamic matching of the thermodynamic path is characterized in that the development path of the thermodynamic states such as reservoir temperature, pressure, gas phase partial pressure and the like is dynamically regulated and controlled to match the initial CH 4 CO in hydrate to development process 2 -N 2 -CH 4 The mixed hydrate phase equilibrium line continuously and safely achieves the thermodynamic conditions of hydrate decomposition and replacement in each production run. The bidirectional enhancement of the kinetic driving force is characterized by timely introduction of CO in the middle and later low-efficiency stage of depressurization or replacement 2 -N 2 The replacement or pressure reduction process continuously utilizes the latter to strengthen the heat and mass transfer (namely bidirectional strengthening) of the former through a pressure reduction-replacement \8230anda pressure reduction-replacement sequence.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that modifications may be made to the embodiments or portions thereof without departing from the spirit and scope of the invention.
Claims (10)
1. A method for exploiting natural gas hydrates from a warm and high-pressure reservoir in a sea area is characterized by comprising the following steps,
drilling one or more groups of double horizontal wells in a hydrate reservoir;
performing staged fracturing on each group of the double horizontal wells;
and (4) alternately carrying out depressurization and gas replacement on the double horizontal wells after the staged fracturing.
2. The method for exploiting natural gas hydrates of warm and high-pressure reservoirs in sea areas according to claim 1,
the hydrate reservoir comprises a sea area warm and high pressure hydrate reservoir, wherein the temperature of the sea area warm and high pressure hydrate reservoir is more than 277K, and the pressure of the sea area warm and high pressure hydrate reservoir is more than 8MPa.
3. A method for producing natural gas hydrates from a warm and high-pressure reservoir in the sea area according to any one of claims 1 and 2,
the dual horizontal well comprises an upper horizontal well and a lower horizontal well, wherein,
the upper horizontal well is arranged at the top of the hydrate reservoir, and the lower horizontal well is arranged at the bottom of the hydrate reservoir.
4. The method for exploiting natural gas hydrates of warm and high-pressure reservoirs in sea areas according to claim 1, wherein the depressurization and gas replacement are alternately carried out on the fractured double horizontal wells, and the method comprises the following steps,
depressurizing the double horizontal wells after the staged fracturing to a preset gas replacement condition;
injecting replacement gas into the lower horizontal well, exhausting the upper horizontal well, and replacing to obtain methane gas;
when the methane obtained is below a first predetermined concentration and the displaced gas is above a second predetermined concentration, or the partial pressure of the displaced gas in the reservoir is increased more than the displaced gas CH in the reservoir 4 When the phase equilibrium pressure of the mixed hydrate is 20 percent, primary pressure reduction and gas replacement are completed;
repeating the steps of reducing the pressure and replacing the gas for N times alternately, wherein N is an integer more than 1.
5. The method for exploiting natural gas hydrates of warm and high-pressure reservoirs in sea areas according to claim 4,
the predetermined gas displacement conditions include a flow rate of wellhead gas and CH 4 Concentration lower than predetermined value, average CH 4 The gas partial pressure is reduced to below 10 percent of the natural gas hydrate phase equilibrium pressure corresponding to the average temperature or the average temperature is reduced by 3-5K compared with the initial temperature of the pressure reduction of the round.
6. The method for exploiting natural gas hydrates of warm high-pressure reservoir in sea area according to claim 5,
the fractured double horizontal wells comprise measuring devices;
the measuring device comprises sensors for temperature, pressure, flow and the like, and is used for monitoring the thermodynamic state of the hydrate at the top and the bottom of the reservoir.
7. The method for exploiting natural gas hydrates of warm and high-pressure reservoirs in sea areas according to claim 5,
the displacement gas comprises CO 2 And N 2 Mixed gas of wherein CO 2 And N 2 In the range of 20:80 to 50 parts of.
8. A sea area warm high-pressure reservoir natural gas hydrate production system is characterized by comprising,
the drilling module is used for drilling a double horizontal well in the hydrate reservoir;
the fracturing module is used for performing staged fracturing on the hydrate reservoir through the double horizontal wells;
and the displacement module is used for alternately carrying out depressurization and gas displacement on the double horizontal wells subjected to staged fracturing.
9. The system for exploiting natural gas hydrates of warm high-pressure reservoirs in sea areas according to claim 8, wherein the fracturing module comprises a depressurization unit and a monitoring unit, wherein,
the depressurization unit is used for carrying out step depressurization on the double horizontal wells to a preset gas replacement condition;
the monitoring unit comprises a temperature sensor, a pressure sensor and a flow sensor and is used for monitoring the flow of the wellhead gas, the methane concentration, the temperature and the pressure of the hydrate reservoir and evaluating the average CH in the hydrate reservoir 4 Gas partial pressure and reservoir average temperature.
10. The method for extracting natural gas hydrates from the sea area warm high-pressure reservoir according to any one of claims 1 to 8, and the application of the method in the extraction of the natural gas hydrates from the sea area warm high-pressure reservoir.
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